1. Field of Invention
The present invention relates generally to reducing the tendency toward aerodynamic stall experienced by aircraft flying at high angles of attack and, more particularly, to placing chordwise extending guides on wings, hydrofoils and control surfaces to suppress vortex-type three-dimensional flow separation over the surface.
2. Brief Description of Related Art
Lifting surfaces such as wings, hydrofoils and control surfaces (hereinafter referred to generally as wings) used on airplanes, missiles and marine vehicles are ordinarily streamlined bodies having smooth surface contours in both the chordwise (leading edge to trailing edge) and spanwise (root to tip) directions. In the chordwise direction, the shape of the wing, as defined by a particular wing section (cross-sectional profile in a plane normal to the wing span), is generally a known airfoil shape. For example, section profiles may be selected from sections of the National Advisory Committee for Aeronautics (NACA) such as NACA 4-digit profiles (e.g., NACA 0020), from NASA LS/MS airfoil series for general aviation, or from alternative section profiles such as TMB-EPH (elliptic-parabolic-hyperbolic) sections for hydrodynamic applications. Thus, the generally streamlined, curvilinear shape of the wing surface is defined by a series of chordwise cross-sections separated in the spanwise direction.
Flow over a wing operating in a fluid medium generates a pressure differential between the upper and lower surfaces of the wing resulting in a lift force. The lift force produced varies with the wing's angle of attack (angle of wing chord relative to the incoming undisturbed free stream flow) and the incoming flow velocity (velocity of wing relative to the undisturbed free stream flow into the wing). Concurrently, a drag force is produced that must be overcome by a thrust force provided by the engine. Streamlined shapes, such as described above, produce favorable aerodynamic or hydrodynamic characteristics at the design point (design angle of attack and design speed). However, those characteristics are degraded significantly at off-design conditions. As the flow accelerates and/or as the angle of attack of the wing increases, flow separation becomes a problem resulting in increased drag and loss of lift.
Based on the fluid dynamics theory of three-dimensional flow separation, two basic types of flow separation in three-dimensions have been identified: bubble-type separation and vortex-type separation. Both types of separation are caused by pressure gradients produced by flow acceleration over the wing surface; however, each type of separation results from entirely different flow mechanisms. Bubble-type separation results when local pressure gradients cause skin friction to diminish. In the case of vortex-type separation, crossflow pressure gradients drive the streamlines in the rear region of the wing to converge in the spanwise direction thus producing a crossflow component of the flow.
During normal operations the wing experiences small to moderate angles of attack. At small angles of attack the streamlines near the body follow the wing surface closely, right to the trailing edge. As the angle of attack increases, changes in flow pattern occur, primarily on the upper surface of the wing. At a certain value of the angle of attack, separation begins at the rear of the wing on the upper side and moves forward as angle of attack increases. Typically, for example, for a particular aircraft design a maximum prestall angle of attack is identified above which the pilot should not operate. Below this angle of attack the lift and drag vary within a known performance envelope. Flow over the wing is predominately chordwise with little or no crossflow or separation. However, there exist circumstances where a vehicle must operate at high angles of attack. For instance, an aircraft undergoing unusual flight maneuvers or engaging in a short take-off and a marine vehicle encountering a sharp turn both experience high angles of attack over wings and control surfaces. As the angle of attack of the wing begins to increase, both lift and drag increase, requiring increased engine power output to overcome the drag. As angle of attack increases further, the flow streamlines in the rear regions of the wing converge in a spanwise direction resulting in vortex-type three-dimensional flow separation and eventually in wing stall, i.e., massive flow separation wherein lift decreases sharply while drag increases sharply. The resulting loss of lift and control produces a highly unstable and dangerous situation. Such flow separation problems have been a major obstacle in the field of aero-hydrodynamics and have spawned extensive research efforts over the past four decades. Consequently, there is a need for a means to increase the operational envelope of known wings, more specifically, a means to increase the angles of attack at which the wing can operate without stalling.